An image output apparatus such as a printer or a CRT forms an image by a set of small pixels. The pixel is normally outputted at a fixed position on a lattice matrix. Therefore, the image of pixels is not represented with accuracy below the interval of the lattice. For example, if an almost perpendicular obligue line 20 is outputted as shown in FIG. 1, noise called jag appeares. The reciprocal number of the lattice interval D30 is called resolution, which is used as the estimation value of the ability of the apparatus to represent images. If the resolution is high, jag noise is decreased. However, in this case, the number of pixels 10 and the amount of data increase. Therefore, cost is high and output speed is low.
As a method to decrease the jag noise without high resolution, a smoothing technique is used. As shown in FIG. 2, smoothing is a technique to smooth a contour by outputting the pixel 40 whose size and position are modulated. However, because of the cost and time required to process pixel data, each pixel of data in the image output apparatus is processed by binary bit map data. Pixel modulated information such as multivalue bit map data is not included in this case. Therefore, in the case of smoothing, it is necessary to generate the smoothing signal of the multivalue pixel modulation data from the binary bit map data.
As a general method to generate the smoothing signal, it is generated by the binary bit map data neighboring the notice pixel to be modulated. The character and line image have characteristics which make the contour continuously smooth. The pixel modulation data and the neighboring data to decrease the jag noise correlate mutually. By using these characteristics, the modulation signal is generated.
As a prior method to recognize the neighboring binary bit map data and calculate the pixel modulation data of the notice pixel, template matching is widely used (Japanese Patent Disclosure (Kokai) H2-112966). In this method, a binary template pattern is matched with a neighboring binary template pattern including the notice pixel of the original image to recognize the neighboring pattern information. The smoothing signal is previously set as the output of each template.
Furthermore, a method to determine the intensity modulation signal by calculating outline information from a bit map image is disclosed in Japanese Patent Disclosure (Kokai) H7-137339. However, this method is not practical because the method to calculate concrete outline information is not disclosed.
Recently, it often happens that image data of various kinds of bit map formats with digitized images is sent to the image output apparatus. However, the density of the image data is sometimes different from that of the image output apparatus. When the image whose resolution is different from the image output apparatus is outputted, the size of the output image is different from the original image size. For example, the image whose resolution is 200 dpi (dot per inch) is outputted through the printer whose resolution is 600 dpi, the image size becomes 1/3 of the original. In order to solve this problem, resolution conversion is used. Concretely speaking, in case the resolution of the image output apparatus is higher than the resolution of the original image, each pixel size of the original image is enlarged. In the reverse case, these are thinned-out. However, in case of enlargement by SPC method, the jag noise is clearly enlarged. Therefore, the enlargement technique of smoothing the contour by using template matching is used.
In the prior method to generate the smoothing modulation signal by the template matching pattern or to convert the resolution, the pattern to convert the modulation signal or the resolution is limited to the number of templates which the designer previously prepared. Therefore, an arbitrary pattern or arbitrary resolution of the image is not coped with and the image quality deteriorates. In order to solve this problem, a large number of templates are prepared and the cost becomes high. Similarly, in a method to calculate the outline of the contour, the processing is complicated and can take a long time.